Skip to main content

Biology · The definition

What Is an Animal?

An animal is a multicellular eukaryotic organism in the kingdom Animalia that obtains energy by consuming other organisms, lacks a cell wall, and — in almost all cases — can move under its own power at some stage of life and develops from a blastula. Around 1.5 million animal species have been described, and the great majority of them are invertebrates.

Updated 25 July 2026

~1.5 million
Animal species formally described by science
7–10 million
Estimated true total, most of them undescribed invertebrates
~95%
Of described animal species are invertebrates — animals without a backbone
~650 million years
Since the earliest animals appear in the fossil record

The definition

An animal is any member of the biological kingdom Animalia, also called Metazoa. The scientific definition rests on cell structure and development rather than on anything you can see from across a room: animals are multicellular, their cells have a nucleus and no rigid wall, they cannot produce their own food, and they pass through a blastula stage as embryos.

That definition is broader than everyday usage. In ordinary speech "animal" often means a large land mammal, and is sometimes used in contrast to people, birds, fish or insects. In biology it includes all of those, plus jellyfish, corals, sponges, tapeworms and sea squirts — and it includes humans without qualification.

The short version

If it is made of many cells, has a nucleus in those cells, has no cell walls, must eat other organisms to live, and moved under its own power at some point in its life — it is an animal.

The seven characteristics of animals

Each trait is listed with its exceptions. A definition that hides its edge cases is not much use — and the exceptions here are where the interesting biology lives.

The seven defining characteristics of animals, with exceptions
Characteristic What it means Exceptions
Multicellular Built from many cells that specialise into tissues and organs. No animal is single-celled. None — this one holds without exception.
Eukaryotic Cells contain a nucleus enclosing the DNA, plus membrane-bound organelles such as mitochondria. None — shared with plants, fungi and protists.
Heterotrophic Cannot make their own food. Animals get energy and carbon by eating other organisms, living or dead. Some corals and sea slugs host photosynthetic algae, but the animal itself still cannot photosynthesise.
No cell walls Animal cells are bounded by a flexible membrane, not a rigid wall. This is what makes animal tissue soft and mobile. None. Plants, fungi and most bacteria have cell walls; animals never do.
Motile at some life stage Able to move under their own power. Movement may be limited to a single stage of the life cycle. Adult sponges, corals and barnacles are fixed in place, but all have free-swimming larvae.
Develop from a blastula The fertilised egg divides into a hollow ball of cells — a blastula — before differentiating. This is a defining feature of animal embryology. None known among animals; the pattern is unique to the kingdom.
Specialised nerve and muscle tissue Nearly all animals have nerve cells for sensing and coordinating, and muscle cells for movement. Sponges and placozoans have neither, and are the earliest-branching animal lineages.

The major animal groups

Around 95% of described animal species are invertebrates. The vertebrates — every mammal, bird, reptile, amphibian and fish that has ever been filmed, photographed or fundraised for — are a small minority of animal life.

Major animal groups by number of described species
Group Type Described species Notes
Insects Invertebrate ~1,000,000 The largest group of animals by an enormous margin — beetles alone account for about 400,000 described species.
Other arthropods Invertebrate ~250,000 Spiders, scorpions, crabs, lobsters, millipedes and centipedes — jointed legs and an external skeleton.
Molluscs Invertebrate ~85,000 Snails, clams, squid and octopuses. Includes the most intelligent invertebrates known.
Fish Vertebrate ~35,000 More species than all other vertebrate groups combined. Not a single evolutionary group but several lineages.
Birds Vertebrate ~11,000 Feathered, warm-blooded, egg-laying — and the only surviving dinosaurs.
Reptiles Vertebrate ~12,000 Snakes, lizards, turtles and crocodilians. Scaled skin and, in most, eggs laid on land.
Amphibians Vertebrate ~8,700 Frogs, salamanders and caecilians. Permeable skin and a life cycle split between water and land — the most threatened vertebrate class.
Mammals Vertebrate ~6,600 Hair or fur, milk-producing mammary glands, and in nearly all, live birth. The smallest major vertebrate group.

Species counts are approximate and change constantly as new species are described and taxonomies revised. Sources: IUCN Red List, Catalogue of Life, and group-specific authorities including BirdLife International and AmphibiaWeb.

Animals compared with plants, fungi and bacteria

How animals differ from plants, fungi and bacteria
Feature Animals Plants Fungi Bacteria
Cell nucleus Yes Yes Yes No
Cell wall Never Cellulose Chitin Peptidoglycan
Gets energy by Ingesting food Photosynthesis Absorbing nutrients Varies
Moves under own power Yes, at some life stage No No Some
Nerve & muscle tissue Nearly all No No No

The result that surprises most people: fungi are more closely related to animals than to plants. Animals and fungi share a common ancestor in a group called Opisthokonta, and both are heterotrophs. The difference is in method — fungi secrete digestive enzymes and absorb the results, while animals take food inside the body and digest it there.

How animals are classified

Every animal sits in a nested hierarchy of ranks, each narrower than the last. Working through it for a single species is the fastest way to see how the system fits together — here is the domestic dog:

  1. 1 Domain: Eukaryota — Cells with a nucleus
  2. 2 Kingdom: Animalia — Animals
  3. 3 Phylum: Chordata — Animals with a notochord — includes all vertebrates
  4. 4 Class: Mammalia — Mammals — hair, milk, live birth
  5. 5 Order: Carnivora — Carnivorans — dogs, cats, bears, seals
  6. 6 Family: Canidae — The dog family — wolves, foxes, jackals
  7. 7 Genus: Canis — Wolves, coyotes, jackals and domestic dogs
  8. 8 Species: Canis lupus — The grey wolf
  9. 9 Subspecies: Canis lupus familiaris — The domestic dog

The two-part scientific name — genus plus species, as in Canis lupus — is called binomial nomenclature, and it is why a species has exactly one unambiguous name worldwide regardless of what it is called locally.

Browse the taxonomic tree Conservation glossary

Where animals came from

  • ~650 million years ago The earliest animals appear. Sponges and comb jellies are the leading candidates for the earliest-branching lineage; which came first is still debated.
  • ~575 million years ago The Ediacaran biota — the first widely accepted animal fossils, including Dickinsonia, soft-bodied organisms unlike anything alive today.
  • ~538 million years ago The Cambrian explosion. Nearly every animal phylum alive today appears in the fossil record within a geologically short window, along with the first hard skeletons, eyes and predators.
  • ~530 million years ago The first vertebrates — small jawless fish, the ancestors of every mammal, bird, reptile, amphibian and fish.
  • ~380 million years ago The first four-limbed vertebrates move onto land, establishing the body plan carried by every amphibian, reptile, bird and mammal since.
  • ~66 million years ago The end-Cretaceous extinction removes the non-avian dinosaurs. Birds survive as the only dinosaur lineage; mammals diversify into the space left behind.

Roughly a quarter of assessed species are threatened

The IUCN Red List has assessed a fraction of the animals that exist, and of those assessed, around 28% are threatened with extinction. Amphibians are worst affected at roughly 41%, followed by sharks and rays, then mammals at about 26% and birds at about 13%.

The animals nobody counts are almost certainly worse off. Invertebrates are 95% of animal species and receive a small fraction of conservation funding and assessment effort — so the headline figures describe the best-studied corner of the problem.

Fund animal protection Animals we have already lost

Explore the animals

What is an animal? Frequently asked questions

What is an animal?
An animal is a multicellular eukaryotic organism belonging to the kingdom Animalia. Animals obtain energy by consuming other organisms rather than producing their own food, have cells without rigid walls, develop from a hollow ball of cells called a blastula, and can move under their own power at some point in their life cycle.
What are the characteristics of animals?
Seven traits define animals: they are multicellular; their cells are eukaryotic, containing a nucleus; they are heterotrophic, meaning they eat other organisms; their cells lack rigid walls; they can move under their own power at some life stage; they develop from a blastula; and nearly all have specialised nerve and muscle tissue.
Are humans animals?
Yes. Humans are animals in every scientific sense — multicellular, eukaryotic, heterotrophic, without cell walls, motile, and developing from a blastula. Humans are mammals in the order Primates, family Hominidae, and share a common ancestor with chimpanzees roughly 6 to 7 million years ago.
What is the difference between animals and plants?
The core difference is how they get energy. Plants are autotrophic — they make their own food by photosynthesis. Animals are heterotrophic and must eat other organisms. Plant cells also have rigid cellulose walls and chloroplasts, which animal cells never have, and most plants cannot move, while nearly all animals can at some life stage.
How many animal species are there?
About 1.5 million animal species have been formally described by science. The true total is estimated at 7 to 10 million, with some estimates far higher once poorly sampled groups such as deep-sea invertebrates, nematodes and parasitic wasps are accounted for. Most undescribed species are invertebrates.
What percentage of animals are invertebrates?
Around 95 to 97 per cent of all described animal species are invertebrates — animals without a backbone. Vertebrates, the group containing every mammal, bird, reptile, amphibian and fish, make up only about 3 to 5 per cent of animal species despite receiving the overwhelming majority of public attention and conservation funding.
Are sponges animals?
Yes. Sponges are animals in the phylum Porifera, despite having no nerves, no muscles, no organs and no ability to move as adults. They are multicellular, eukaryotic and heterotrophic, they lack cell walls, and their larvae swim freely — which satisfies the definition. They are among the earliest-branching animal lineages.
What was the first animal on Earth?
The earliest animals appear in the fossil record around 600 to 650 million years ago, in the Ediacaran period. Sponges and comb jellies are the leading candidates for the earliest-branching animal lineage, and which came first is still actively debated. The first widely accepted animal fossils are Ediacaran biota such as Dickinsonia.
Is a jellyfish an animal?
Yes. Jellyfish are animals in the phylum Cnidaria, related to corals and sea anemones. They have no brain, no bones and no blood, and are roughly 95 per cent water, but they are multicellular heterotrophs with nerve and muscle tissue that move under their own power.
What makes an animal different from a fungus?
Both are multicellular heterotrophs, and fungi are in fact more closely related to animals than to plants. The differences are structural: fungi have cell walls made of chitin, absorb nutrients by secreting digestive enzymes externally rather than ingesting food, and cannot move under their own power.